Froth Flotation
Industrially, processes for the purification of liquid suspensions or dispersions (and especially aqueous suspensions or dispersions) to remove suspended solid particles are quite prevalent. Froth flotation, for example, is a separation process based on differences in the tendency of various materials to associate with rising air bubbles. Additives are often incorporated into the froth flotation liquid (e.g., aqueous brine) to improve the selectivity of the process. For example, “collectors” can be used to chemically and/or physically absorb onto mineral(s) to be floated, rendering them more hydrophobic. On the other hand, “depressants,” typically used in conjunction with collectors, render other materials (e.g., gangue minerals) less likely to associate with the air bubbles, and therefore less likely to be carried into the froth concentrate.
In this manner, some materials (e.g., value minerals) will, relative to others (e.g., gangue materials), exhibit preferential affinity for air bubbles, causing them to rise to the surface of the aqueous slurry, where they can be collected in a froth concentrate. A degree of separation is thereby effected. In less common, so-called reverse froth flotations, it is the gangue that is preferentially floated and concentrated at the surface, with the desired materials removed in the bottoms. Gangue materials typically refer to quartz, sand and clay silicates, and calcite, although other minerals (e.g., fluorite, barite, etc.), may be included. In some cases, the material to be purified (i.e., the desired material) actually comprises predominantly such materials, and the smaller amounts of contaminants are preferentially floated. For example, in the beneficiation of kaolin clay, a material having a number of industrially significant applications, iron and titanium oxides can be separated by flotation (into a froth concentrate) from the impure, clay-containing ore, leaving a purified kaolin clay bottoms product.
The manner in which known collectors and depressants achieve their effect is not understood with complete certainty, and several theories have been proposed to date. Depressants, for example may prevent the gangue minerals from adhering to the value materials to be separated, or they may even prevent the collector(s) from absorbing onto the gangue minerals. Whatever the mechanism, the ability of a depressant to improve the selectivity in a froth flotation process can very favorably impact its economics.
Overall, froth flotation is practiced in the beneficiation of a wide variety of value materials (e.g., mineral and metal ores and even high molecular weight hydrocarbons such as bitumen), in order to separate them from unwanted contaminants which are unavoidably co-extracted from natural deposits. A particular froth flotation process of commercial significance involves the separation of bitumen from sand and/or clay, which are ubiquitous in oil sand deposits, such as those found in the vast Athabasca region of Alberta, Canada. Bitumen is recognized as a valuable source of “semi-solid” petroleum or heavy hydrocarbon-containing crude oil, which can be upgraded into many valuable end products including transportation fuels such as gasoline or even petrochemicals. Alberta's oil sand deposits are estimated to contain 1.7 trillion barrels of bitumen-containing crude oil, exceeding the reserves in all of Saudi Arabia. For this reason, significant effort has been recently expended in developing economically feasible operations for bitumen recovery, predominantly based on subjecting an aqueous slurry of extracted oil sand to froth flotation. For example, the “Clark Process” involves recovering the bitumen in a froth concentrate while depressing the sand and other solid impurities.
Various gangue depressants for improving froth flotation separations are known in the art and include sodium silicate, starch, tannins, dextrins, lignosulphonic acids, carboxyl methyl cellulose, cyanide salts and many others. More recently certain synthetic polymers have been found advantageous in particular beneficiation processes using froth flotation. For example, U.S. Pat. No. Re. 32,875 describes the separation of gangue from phosphate minerals (e.g., apatite) using as a depressant a phenol-formaldehyde copolymer (e.g., a resol, a novolak) or a modified phenol polymer (e.g., a melamine-modified novolak).
U.S. Pat. No. 3,990,965 describes the separation of iron oxide from bauxite using as a depressant a water soluble prepolymer of low chain length that adheres selectively to gangue and that can be further polymerized to obtain a cross-linked, insoluble resin.
U.S. Pat. No. 4,078,993 describes the separation of sulfide or oxidized sulfide ores (e.g., pyrite, pyrrhotite, or sphalerite) from metal mineral ores (e.g., copper, zinc, lead, nickel) using as a depressant a solution or dispersion of a low molecular weight condensation product of an aldehyde with a compound containing 2-6 amine or amide groups.
U.S. Pat. Nos. 4,128,475 and 4,208,487 describe the separation of gangue materials from mineral ore using a conventional frothing agent (e.g., pine oils) combined with a (preferably alkylated) amino-aldehyde resin that may have free methylol groups.
U.S. Pat. No. 4,139,455 describes the separation of sulfide or oxidized sulfide ores (e.g., pyrite, pyrrhotite, or sphalerite) from metal mineral ores (e.g., copper, zinc, lead, nickel) using as a depressant an amine compound (e.g., a polyamine) in which at least 20% of the total number of amine groups are tertiary amine groups and in which the number of quaternary amine groups is from 0 to not more than ⅓ the number of tertiary amine groups.
U.S. Pat. No. 5,047,144 describes the separation of siliceous materials (e.g., feldspar) from minerals (e.g., kaolinite) using as a depressant a cation-active condensation product of aminoplast formers with formaldehyde, in combination with cation-active tensides (e.g., organic alkylamines) or anion-active tensides (e.g., long-chained alkyl sulfonates).
Russian Patent Nos. 427,737 and 276,845 describe the depression of clay slime using carboxymethyl cellulose and urea-formaldehyde resins, optionally combined with methacrylic acid-methacrylamide copolymers or starch ('845 patent).
Russian Patent Nos. 2,169,740; 2,165,798; and 724,203 describe the depression of clay carbonate slimes from ores in the potassium industry, including sylvinite (KCl—NaCl) ores. The depressant used is a urea/formaldehyde condensation product that is modified by polyethylenepolyamine. Otherwise, a guanidine-formaldehyde resin is employed ('203 patent).
Markin, A. D., et. al., describe the use of urea-formaldehyde resins as carbonate clay depressors in the flotation of potassium ores. Study of the Hydrophilizing Action of Urea-Formaldehyde Resins on Carbonate Clay Impurities in Potassium Ores, Inst. Obshch. Neorg. Khim, USSR, Vestsi Akademii Navuk BSSR, Seryya Khimichnykh Navuk (1980); Effect of Urea-Formaldehyde Resins on the Flotation of Potassium Ores, Khimicheskaya Promyshlennost, Moscow, Russian Federation (1980); and Adsorption of Urea-Formaldehyde Resins on Clay Minerals of potassium Ores, Inst. Obshch Neorg. Khim., Minsk, USSR, Doklady Akademii Nauk BSSR (1974).
As is recognized in the art, a great diversity of materials can be subject to beneficiation/refinement by froth flotation. Likewise, the nature of both the desired and the unwanted components varies greatly. This is due of the differences in chemical composition of these materials, as well as in the types of prior chemical treatment and processing steps used. Consequently, the number and type of froth flotation depressants is correspondingly wide.
Also, the use of a given depressant in one service (e.g., raw potassium ore beneficiation) is not a predictor of its utility in an application involving a significantly different feedstock (e.g., bitumen-containing oil sand). This also applies to any expectation regarding the use of a depressant that is effective in froth flotation, in the any of the separations of solid contaminants from aqueous liquid suspensions, as described below (and vice versa). The theoretical mechanisms by which froth flotation and aqueous liquid/solid separations occur are significantly different, where the former process relies on differences in hydrophobicity and the latter on several other possibilities (charge destabilization/neutralization, agglomeration, host-guest theory (including podands), hard-soft acid base theory, dipole-dipole interactions, Highest Occupied Molecular Orbital-Lowest unoccupied Molecular Orbital (HOMO-LUMO) interactions, hydrogen bonding, Gibbs free energy of bonding, etc). Traditional depressants in froth flotation for the benefication of metallic ores, such as guar gum, are not employed as dewatering agents, or even as depressants in froth flotation for bitumen separation. Moreover, in two of the applications described below (waste clay and coal dewatering), no agents are currently used to improve the solid/liquid separation. Overall, despite the large offering of flotation depressants and dewatering agents in the art, an adequate degree of refinement in many cases remains difficult to achieve. There is therefore a need in the art for agents which can be effectively employed in a wide range of separation processes, including both froth flotation and the separation of solid contaminants from liquid suspensions.
Other Separations
Other processes, in addition to froth flotation, for the separation of solid contaminants from liquid suspensions can involve the use of additives that either destabilize these suspensions or otherwise bind the contaminants into larger agglomerates. Coagulation, for example, refers to the destabilization of suspended solid particles by neutralizing the electric charge that separates them. Flocculation refers to the bridging or agglomeration of solid particles together into clumps or flocs, thereby facilitating their separation by settling or flotation, depending on the density of the flocs relative to the liquid. Otherwise, filtration may be employed as a means to separate the larger flocs.
The additives described above, and especially flocculants, are often employed, for example, in the separation of solid particles of rock or drill cuttings from oil and gas well drilling fluids. These drilling fluids (often referred to as “drilling muds”) are important in the drilling process for several reasons, including cooling and lubricating the drill bit, establishing a fluid counterpressure to prevent high-pressure oil, gas, and/or water formation fluids from entering the well prematurely, and hindering the collapse of the uncased wellbore. Drilling muds, whether water- or oil-based, also remove drill cuttings from the drilling area and transport them to the surface. Flocculants such as acrylic polymers are commonly used to agglomerate these cuttings at the surface of the circulating drilling mud, where they can be separated from the drilling mud.
Other uses for flocculants in solid/liquid separations include the agglomeration of clays which are suspended in the large waste slurry effluents from phosphate production facilities. Flocculants such as anionic natural or synthetic polymers, which may be combined with a fibrous material such as recycled newspaper, are often used for this purpose. The aqueous clay slurries formed in phosphate purification plants typically have a flow rate of over 100,000 gallons per minute and generally contain less than 5% solids by weight. The dewatering (e.g., by settling or filtration) of this waste clay, which allows for recycle of the water, presents one of the most difficult problems associated with reclamation. The settling ponds used for this dewatering normally make up about half of the mined area, and dewatering time can be on the order of several months to several years.
In the separation of solids from aqueous liquids, other specific applications of industrial importance include the filtration of coal from water-containing slurries (i.e., slurry dewatering), the treatment of sewage to remove contaminants (e.g., sludge) via sedimentation, and the processing of pulp and paper mill effluents to remove suspended cellulosic solids. The dewatering of coal poses a significant problem industrially, as the BTU value of coal decreases with increasing water content. Raw sewage, both industrial and municipal, requires enormous treatment capacity, as wastes generated by the U.S. population, for example, are collected into sewer systems and carried along by approximately 14 billion gallons of water per day. Paper industry effluent streams likewise represent large volumes of solid-containing aqueous liquids, as waste water generated from a typical paper plant often exceeds 25 million gallons per day. The removal of sand from aqueous bitumen-containing slurries generated in the extraction and subsequent processing of oil sands, as described previously, poses another commercially significant challenge in the purification of aqueous liquid suspensions. Also, the removal of suspended solid particulates is often an important consideration in the purification of water, such as in the preparation of drinking (i.e., potable) water. Synthetic polyacrylamides, as well as naturally-occurring hydrocolloidal polysaccharides such as alginates (copolymers of D-mannuronic and L-guluronic acids) and guar gum are flocculants in this service.
The above applications therefore provide several specific examples relating to the treatment of aqueous liquid suspensions to remove solid particulates. However, such separations are common in a vast number of other processes in the mineral, chemical, industrial and municipal waste, sewage treatment, and paper industries, as well as in a wide variety of other water-consuming industries. Thus, there is a need in the art for additives that can effectively promote selective separation of a variety of solid contaminants from liquid suspensions. Advantageously, such agents should be selective in chemically interacting with the solid contaminants, through coagulation, flocculation, or other mechanisms such that the removal of these contaminants is easily effected. Especially desirable are additives that are also able to complex unwanted ionic species such as metal cations to facilitate their removal as well.